Conventionally, many members such as suspension members or body structural members of an automobile are manufactured by pressing a steel sheet that has predetermined strength. In recent years, from a viewpoint of global environmental conservation, the weight saving of an automobile body is desired, and efforts are being made to reduce the steel-sheet thickness by increasing steel-sheet strength. However, because the press workability of steel sheet is deteriorated with an increase in the strength of the steel sheet, it is difficult to form the steel sheet into a desired shape in many cases.
Patent Literature 1 discloses a pressing technique called hot pressing. This technique realizes both easy pressing and high strength by quenching a heated steel sheet at the same time when the heated steel sheet is pressed by using a mold formed of a die and a punch. However, in this hot pressing, because the steel sheet is heated to the high temperature of around 950° C. before hot pressing, scales (iron oxide) are formed on the surface of the steel sheet. The scales formed on the surface of the steel sheet are peeled off during hot pressing and hence, the mold is damaged or the surface of the member is damaged after hot pressing. Furthermore, the scales remaining on the surface of the member cause a defect in appearance and a defect in painting adhesion. Accordingly, the scales formed on the surface of the member are usually removed by pickling or shot-blasting. These additional processes make manufacturing processes complicated thus causing deterioration in productivity. Furthermore, the members such as suspension members and body structural members of the automobile also require excellent anti-corrosion characteristics. However, an anti-corrosive coating such as a plating layer is not applied to hot-pressed members manufactured in the above-mentioned processes and hence, the anti corrosion characteristics of the members are extremely insufficient.
In such a background, a hot pressing technique capable of suppressing formation of the scales during heating before hot pressing and improving the anti-corrosion characteristics is required. Therefore, a steel sheet to which the anti-corrosive coating such as the plating layer is applied and a method for hot-pressing a member using the steel sheet are proposed. For example, Patent Literature 2 discloses a manufacturing method for a hot-pressed member having excellent anti-corrosion characteristics. This manufacturing method includes a step of hot-pressing a steel sheet coated with Zn or Zn-based alloy and a step of providing a Zn—Fe-based compound or a Zn—Fe—Al-based compound on the surface of the heated steel sheet.